US 2014/0163889 A1 discloses a method of acoustic surveying, wherein distributed optical fibre sensors are used for distributed acoustic sensing wherein a modal analysis of distributed acoustic data obtained in-well is performed for monitoring well integrity. One or more acoustic modes corresponding to distributed speed of sound measurements within the wellbore are performed.
US 2010/0158431 A1 discloses an optical fibre surveillance topology including a single optical fibre for the detection of acoustic signals associated with vibrations or other activity. The fibre is arranged in a two-dimensional topology to improve the spatial resolution of the system. Two or more such oriented fibres may be juxtaposed or overlapped to yield even greater resolution.
US 2015/0346370 A1 discloses a method of calibration for downhole fibre optic distributed acoustic sensing, wherein a vibration tool is positioned at a specified depth in the borehole and two or more distributed acoustic sensing signals are obtained while the vibration tool is activated. The distributed acoustic sensing system is calibrated based on the two or more distributed acoustic sensing signals, the specified depth, the respective specific frequency, and the respective specified amplitude. The method may also comprise disposing two or more geophones at different locations within the borehole to obtain the baseline distributed acoustic sensing signal and the calibration distributed acoustic sensing signal. The vibration tool may be moved to one or more other known depths and the process of obtaining distributed acoustic sensing signals may be repeated.
WO 2014/088786 A1 discloses a calibration of a well acoustic sensing system wherein predetermined acoustic signals along acoustic sensors distributed proximate a well are received and the system is calibrated based on the received acoustic signals. An acoustic source is displaced along an optical waveguide positioned proximate a well, predetermined acoustic signals are transmitted from the acoustic source and the acoustic signals are received with the waveguide and the system is calibrated based on the received acoustic signals. An active sound source is dropped, injected or lowered by cable into a well bore for the purpose of calibrating an optical distributed acoustic sensor. Various vibration speakers, vibrating actuators and acoustic transducers are capable of actively producing sounds within an object. Acoustic signals at several acoustic frequencies as well as extent of the acoustic signals may be used. A measurement of the intensity of the sound energy provides the acoustic sensitivity as a function of position along the distributed acoustic sensor. The sounds can be emitted as continuous single-frequency tones, continuous dual tone multiple frequency, continuous multiple-frequency tones, continuous wide spectrum tones, continuously wide noise, continuous coloured noise, continuously repeating swept-frequency waveforms, continuous pseudo-random waveforms or other continuously repeating complex waveforms. A sensitivity of the waveguide to acoustic energy can depend significantly on how the waveguide is installed in the well and on local variations such as liquids in the well bore. In a calibration procedure, these variations can be compensated for by detecting predetermined acoustic signals transmitted along the waveguide by an acoustic source. By emitting sound in a controlled manner, the distributed acoustic sensing system can measure the acoustic sensitivity as a function of position along the waveguide. The calibration method may include the use of a remote vibratory or impulse seismic source and a calibrated reference receiver such as a three-axis geophone placed adjacent to the distributed acoustic sensor, wherein the calibrated reference receiver is not required, but will improve the accuracy of the calibration by accounting for the signal attenuation and distortion effects caused by the formation between the source and the distributed acoustic sensing sensor. The seismic source can be located either on the surface or in a nearby well. Acoustic signals may be transmitted from, at or near the earth surface.
It has however been observed that accuracy and/or reliability of distributed acoustic sensing results are not in all condition satisfactory, in particular due to improper calibration.
Thus, there may be a need for a method for calibrating a distributed fibre optic, in particular acoustic, sensing system and for an improved distributed fibre optic, in particular acoustic, sensing system being adapted to be properly calibrated.